QoL: Shift+Enter save-and-continue + autofocus quick-add on landing

- Shift+Enter in the quick-add title/body saves the current note and immediately
  starts a fresh one, staying focused for rapid capture. Plain Enter is still a
  newline (body) / title→body (title). Placeholder hints the shortcut.
- Landing on the board auto-opens + focuses the note input (QuickAdd autofocus
  prop, passed from BoardView) so you can start typing on arrival.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/views/BoardView.vue

@@ -112,7 +112,7 @@ async function signOut() {
     </header>
 
     <main class="mx-auto w-full max-w-6xl flex-1 px-4 py-6">
-      <QuickAdd v-if="currentView === 'active'" class="mb-8" />
+      <QuickAdd v-if="currentView === 'active'" autofocus class="mb-8" />
 
       <div v-if="notes.loading" class="py-24 text-center text-sm text-neutral-400">Loading…</div>